1945
Ind. Eng. Chem. Res. 1991,30, 1945-1949 Wagner, Z.;Wichterle, I. High-PreeeureVapour-Liquid Equilibrium in Systems Containing Carbon Dioxide, 1-Helene, and n-Hexane. Fluid Phase Equilib. 1987,33, 109-123. Weber, W.; Zeck, S.; Knapp, H. Gas Solubilities in Liquid Solvents at High Pressures: Apparatus and Reaulta for Binary and Ternary Systems of N2, COSand CH,OH. Fluid Phase Equilib. 1984,18, 253-278. Zeck, S.; Knapp, H. Vapor-Liquid and Vapor-Liquid-Liquid Phase Equilibria for Binary and Ternary Systems of Nitrogen, Ethane
and Methanol: Experiment and Data Reduction. Fluid Phase Equilib. 1986,25,303-322. Zeck, S.; Knapp, H. Vapor-Liquid and Vapor-Liquid-Liquid Phase Equilibria of Binary and Ternary Systems of Nitrogen, Ethene and Methanol: Experiment and Data Evaluation. Fluid Phase Equilib. 1986,26, 37-58.
Receiued for reuiew October 24, 1990 Accepted March 13, 1991
Combination of Microtomy and NMR Spectroscopy To Probe the Variation in Horizontal Layers of Bondlines Louis
F.Cannizzo
Aduanced Technology, MIS 244, Thiokol Corporation, P.O.Box 707, Brigham City, Utah 84302
A technique was developed that provides information on the chloroform-soluble components in the horizontal layers of bondlines. The bondlines examined were in samples that consisted of a mat of polyaramid fibers, in an epoxy prepreg matrix, bonded to a layer of rubber. The sectioning, with a microtome instrument, of the bondlines (ca. 250 pm in thickness) yielded slices 20 pm in thickness. Approximately 12 slices/bondline were obtained. The chloroform-soluble fraction of these slices was analyzed by proton NMR spectroscopy to determine the chemical species present. This information was combined with the location of the slices to give a horizontal analysis of the concentrations of the different chloroform-soluble components present across the thicknew of the bondline. The results obtained indicated the degree of cure of the different components of the original bondline, the migration of these components, and the migration of the plasticizer dioctyl phthalate (DOP) from the rubber layer into the bondline. Introduction Analysis of the chemistry of adhesive bondlines has relied on several methods to determine the degree of cure and chemical composition of the cured adhesive. The degree of cure can be measured by the quantity of soluble, uncured material present after cure and by other methods such as differential scanning calorimetry and dynamic mechanical analysis. Chemical tests can quantify the amount of unreacted epoxy groups and other functionalities present after curing. Infrared and NMR spectroscopy, chromatography, and elemental analysis are also useful tools for bondline analysis. Reviews have appeared recently by Jahn and Goetzky (1988)and Hadad (1988)of the various analytical methods available. However, more detailed information concerning selective migration of different components of the adhesive, migration of plasticizers from the materials bonded together into the bondline, and the degree of cure in different horizontal layers o f the bondline is not directly obtained by the above methods. A recent manufacturing investigation required the detailed information described above. We developed a combined technique of microtomy and Nh4R spectroscopy in the analysis of laboratory bondline samples to give the detailed description of bondlines we required. Microtomy is routinely used in many applications that require the sectioning of solid materials into very thin slices (ca. 1-50 pm in thickness) for further analysis. Ultramicrotomy, producing slices of ca. 300-600 A in thickness, coupled with transmission electron microscopy has been utilized to examine the polymer-metal interface in compooite-coated metal samples by Dartyge and Ferrieres (1980)and polymer-metal bonds by Schaefer et al. (1986). Similar analytical techniques have been employed by Wolf (1978)to measure structure, shape, and flaws for quality control in
plastics manufacturing and by Stangl et al. (1976)to evaluate the aging of high-density polyethylene and ABS copolymer samples at different depths. Also rubber blendability has been analyzed with these techniques by Kruse (1976). We utilized standard microtomy on bondline samples to give individual horizontal slices 20 pm in thickness. The thinness of these slices made possible the collection of a sufficient number of samples per bondline in order to be able to obtain a detailed cross-sectional picture of the chemical species present. NMR analysis of the chloroform-soluble fraction of the slices provided the concentrations of the different soluble components. The studies reported herein involve both the development of the technique and the results obtained. Experimental Section Preparations of Bondline Samples. The bondline samples were prepared at Advanced Technology, Thiokol. Diagrams representing the configurations of the samples are presented in Figure 1. Flat panels (15 cm X 15 cm X 1.3 cm) of Kevlar (a registered trademark of Dupont) fibers, containing an epoxy prepreg matrix (resin l),were laminated, and one side was coated with ca. 0.25-mm layer of an additional amount of resin 1. A second different adhesive layer (resin 2) (held in a Dacron (a registered trademark of Dupont) mesh) was placed on top of the layer of resin 1. This two-resin system (resins 1 and 2)gives a better bond than just resin 1by itself. The next layer of the samples consisted of V-45 NBR rubber (Kirkhill) (2.5 mm in thickness). The rubber layers employed were preconditioned to give samples with two different moisture contents. This was accomplished by preconditioning at 0 and 60% relative humidity for 24 h, respectively. Half of one side of each rubber layer (both Ydry"and "wet"
OSSS-5SS5/91/2630-1945$02.50/0 1991 American Chemical Society
1946 Ind. Eng. Chem. Res., Vol. 30, No.8, 1991
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layer of NBR rubber (ca.0.1 "in thickness) resin t 3 (coated on NBR rubber) resln #2
, - * resin #I
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original sample before water jet cutting (cross-sectionalview)
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-
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Figure 1. Configurations of the two different bonding systems employed.
samples) was coated with an epoxy resin (resin 3; a different resin than that employed for the Kevlar fibers) before placing the rubber layer (epoxy resin side down) on top of the layer of resin 2. The other half of the same side of the rubber layer was left uncoated. The assembled samples were placed in vacuum bags and held at ambient temperature for 24 h. They were then cured for 55 h at 48-97 "C under 3.4 atm of pressure. After being cured, the flat panels were cut, by employing a high-pressure water-jet cutter, to give four 6.4 cm X 5.1 cm X 1.5 cm samples for each panel. This yielded two sets of identical samples from each panel. One set contained two different resins in the bondline (two-resin samples; resins 1and 2) while the other set contained three resins (three-resin samples; resins 1-3). Resin 3 was from the epoxy resin coated on half of one side of each rubber layer. This provided two identical bondline samples for each set of parameters. The additional resin (resin 3)in the second set of samples was employed on the basis of prior experience with this resin that showed improved bonding of similar rubber layers to other materials. Inspection of the bondlines by microscopy gave a nominal thickness of 250 Clm. Microtomy. The microtomy was performed at the Harrington Arthritis Research Center in Phoenix, AZ. A Polycut F microtome instrument (Cambridge Instruments) equipped with a tungsten carbide blade was employed. The rubber layer was removed first in slices of ca. 50 pm in thickness. As the bondline was approached, thinner slices were removed. At the inception of the bondline (visual inspection) slices 20 pm in thickness were collected. The f i t few slices obtained were rubbery in texture. The additional slices collected were more brittle. After approximately a dozen slices were collected, the Kevlar fibers were reached and the microtomy terminated. The weight of each slice was recorded along with its position. NMR Spectroscopy. The individual slices of the bondlines obtained were allowed to stand in 2.0 mL of CDCl, overnight. The supernatant was analyzed by proton NMR spectroscopy employing a Varian Associates XL-300 spectrometer. Spectral data are referenced to residual chloroform.
Figure 2. 300 MHz 'H NMR spectrum (in CDCl,) of the chloroform-soluble fraction of slice 7 from the two-resin (wet) sample.
Analysis of NMR Data. The 'H NMR spectra of the chloroform-soluble fraction of the slices obtained from microtomy indicated a complex mixture of compounds were present. A representative spectrum is given in Figure 2. On the basis of a comparison of these spectra with the spectra obtained of the individual components, the following compounds were identified: Dacron, dioctyl phthalate (DOP), partially cured diglycidyl ether bisphenol A (DGEBA), poly(butadiene-co-acrylicacid-co-acrylonitrile) (PBAN), a proprietary cure agent, and partially cured diglycidyl ester of the hydrogenated dimer of linoleic acid (Epon 871,Shell). Analysis of the integration of the peaks of the spectra provided the relative amounts of each component present. The relative amount of soluble Dacron was determined by integration of the aromatic (8.1ppm, four protons) and aliphatic (4.7ppm, four protons) signals and averaging of the two values. Integration of the aromatic region of DOP (7.5-7.7ppm, four protons) gave the relative amount of this component. The aromatic protons of the bisphenol A portion of partially cured DGEBA appeard as a separate group of peaks at 6.7-7.2 ppm (eight protons). Integration of these peaks was used to calculate the total amount of partially cured DGEBA present. The olefinic protons of PBAN appeared at 4.8-5.6 ppm. By analysis of samples with known weights of PBAN, a conversion factor of olefinic integration to relative weight percent was determined. This conversion factor was applied to give the relative amounts of PBAN in the NMR samples. The signals for the aliphatic ether unit of partially cured DGEBA (3.5-4.5 ppm, five protons) overlapped the signals arising from the methylene protons adjacent to the oxygen atoms in DOP (4.2ppm, four protons). The spectra also contained other minor unidentified peaks in the 3-4 ppm region. The aliphatic signal of the proprietary cure agent appeared at 3.0 ppm, and its integration gave the amount of this component. The remainder of the spectra consisted of signals at 2.0-2.7 ppm (aliphatic signals of PBAN), 1.6 ppm (methyl protons of bisphenol A), and 0.8-1.3 ppm. The last set of signals (0.8-1.3ppm) were due to both the aliphatic protons of DOP and Epon 871. The two major peaks of the region were assigned to DOP (1.3ppm) and Epon 871 (1.2ppm), on the basis of the reference spectrum of each compound. Mixtures of DOP and Epon 871 containing different weight percents of the two components
Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991 1947 were prepared and analyzed by ‘HNMR spectroscopy. A calibration curve of the relative heights of the major peaks to the relative weight percent of the two components was prepared and used in determining the amount of Epon 871 in the experimental samples. A known weight of 1,1,2,2-tetrachloroethane(appearing in the spectra as a singlet at 6.0 ppm) was added to the NMR samples before analysis to provide an internal standard for the determination of the weights of each component present. The weight percent (wt %) values reported for each component (see Results and Discussion) are the weight fraction of the entire slice (soluble and nonsoluble). With the known sample weight of each slice and experimentally determined weight of soluble components, a weight percent soluble (% sol.) value (the sum of the weight percentages of the individual soluble components) was also calculated for each sample. To provide a horizontal analysis of the concentrations of the different soluble components in the bondline of a sample, the results from the NMR analysis of each slice were combined to give concentrations for five larger artificial slices of equal weight. Plotting of these values on the same graph produced the final cross-sectional picture of the concentration gradients of the different soluble species present in the bondline. With the approximations employed during the entire analysis and the inherent error of NMR integration as an analytical method, the final values obtained are estimated to have a relative error of f10% of the total value. Weight Percent Soluble Analysis. The bondlines of samples were also examined by a standard Thiokol procedure for determining the percentage of chloroform-soluble material. The first step was removal of the NBR rubber layer with a mechanical grinder. The resulting bondline surface was carefully scraped with a metal scalpel to remove all of the bondline material before encountering the Kevlar fibers. The material obtained (ca. 200 mg) was allowed to stand in 10 mL of chloroform overnight. The resulting gel obtained was washed with an additional 15 mL of chloroform and evacuated in a vacuum oven until a constant weight was obtained. The combined supernatant and washings were evacuated to dryness to give the soluble fraction, which was analyzed by ‘H NMR spectroscopy. The % sol. was determined from the individual weights of the soluble and insoluble fractions.
Results and Discussion As discussed and reviewed by Hadad (1988), the polymerization of epoxy-based resins containing a mixture of components can be very complex in nature. The chemical reactions involved are complicated by selective migration of the components, differing reactivities of the different components, and changes in diffusivities and reactivities as the polymerization proceeds and the viscosity of the resin increases. The information discussed in the following pages details only the composition of the chloroform-soluble (partially cured) components of the bondline of the cured resin. This provides a partial, but still useful, picture of the chemistry of the bondline. Two different bonding systems were studied (see Experimental Section). The first set of samples consisted of the combination of two different adhesive systems, while the second had an additional resin present. Samples of both systems were made with both dry and wet rubber layers in order to study the effect of moisture in the rubber layer on the curing of the bondline. The three-resin system was projected to give a better cure in the presence of moisture, on the basis of prior experience with resin 3. In the following discussion, the two-part adhesive system
samples are designated as “two-resin (dry)”and “two-resin (wet)” (dry and wet rubber layers, respectively) while the three-part adhesive system samples are referred to as “three-resin (dry)” and “three-resin (wet)”. The cross-sectional concentrations of three of the six soluble components observed in the analysis of the samples (see Experimental Section, Analysis of NMR Data) are given in Figure 3. These soluble components (DGEBA, DOP, and Dacron) were observed in each bondline system. They were selected for the following discussion because their concentration profiles illustrate the different properties of bondlines that can be measured by microtomy/ NMR analysis. Both series of systems (two-resin and three-resin) showed increases in soluble DGEBA for the wet rubber layer samples compared to that for the dry rubber samples. The concentration gradient of soluble DGEBA for the three-resin samples displayed lower values at all points across the bondline compared to those of the corresponding twc-resin samples with rubber layers of the same humidity preconditioning, indicating a better cure for the former systems. The two-resin samples also had higher levels of soluble DGEBA next to the rubber layer compared to the levels next to the Kevlar layer. The concentration profiles of DOP in the samples indicated a significant difference between the two-resin and three-resin samples, with lower levels observed for the latter. Both systems showed higher concentrations of DOP next to the rubber layer as one would expect, on the basis of migration of DOP from the rubber layer into the bondline. However, the extra resin next to the rubber layer in the three-resin samples, resin 3, appears to hinder the migration of DOP into the bondline of these samples. One additional point is that the moisture level of the rubber layer did not affect the migration of DOP in either set of samples. Resin 2 (employed in both systems) contained a Dacron mesh as a carrier. A portion of the Dacron was chloroform-soluble and provided a marker across the bondline for all of the samples. These concentation profiles are similar for each sample, which implies that the determination of concentration gradients by microtomy/NMR spectroscopy is a very reproducible technique. The combination of all of the concentration profiles yielded a profile of the 9% sol. across the bondlines of the samples examined (Figure 4). The three-resin samples both display a fairly constant concentration of soluble components across the bondline, with the dry sample giving lower values. In contrast, the two-resin samples show a gradient, with higher concentrations next to the rubber layer and lower values next to the Kevlar. The overall 5% sol. for the two-resin samples was greater than that for the three-resin samples. The % sol. as a measure of the degree of cure is complicated by the presence of uncurable components (Dacron and DOP). Correcting for these materials yields a corrected % sol. (Figure 5). The corrected % sol. profiles for the different samples exhibit the same trends observed in the uncorrected % sol. profiles. With use of the profiles as a measure of the degree of cure, it is obvious that the curing of the two-resin samples was adversely affected by the rubber layer when compared to the curing of the three-resin samples. One possible explanation is that the resin layer in the threeresin samples effectively blocked the moisture present in the rubber layer from entering the bondline. The threeresin (wet) sample still displayed an increase in % sol. compared to the three-resin (dry) sample, which indicated some moisture was still entering the bondline. A better explanation is that the excellent cure chemistry of the extra
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resin (resin 3), demonstrated by the absence of two of the three components of this resin in the soluble fraction of the bondline material, helped the cure of the remainder of the bondline. One effect that was not observed in the concentration gradients was the expected increases in the concentrations of characteristic components for a resin at the location where the resin was orginally placed. For example,
Figure 6. Relative weight percentages of the soluble components from the samples analyzed by the standard % sol. analysis (determined by 'H NMR spectroscopy).
characteristiccomponents used in the Dacron-carried resin system appeared across the entire bondline and did not show greater concentrations at the region where this resin was applied. This implies that during the curing of the samples the different components migrated freely in the bondline. The two characteristic components of the epoxy resin coated on the rubber layer of half of the samples (resin 3) were also not observed, so that, overall, there were no differences in which components were observed in the two-resin and three-resin systems. To verify the validity of the microtomy/NMR technique for bondline analysis, identical bondline samples were examined by a standard % sol. analysis (see Experimental Section). The soluble portion of these samples were analyzed by 'H NMR spectroscopy and the relative amounts of the different components determined as before. These results are presented in Figure 6. The microtomy/NMR results were combined to give one overall value for each
Ind. Eng. Chem. Res., Vol. 30,No. 8, 1991 1949 two-resin (dry) two-resin (wet) three-resin (dry) three-resin (wet) CI
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Conclusions In the bondline systems investigated in these studies it was determined that the degree of cure next to the softer rubber interface was lower and increased across the bondline. Characteristic components from individual resins were found to have migrated between the horizontal layers of the bondline. The migration of the plasticizer DOP from the rubber layer into the bondline was also observed. Comparison of the microtomy/NMR values with the traditional standard % sol. analysis indicates the two methods give comparable bulk values with much more information available from the microtomy/NMR technique. In summary, the combination of microtomy and NMR spectroscopy for the analysis of the bondline yields a detailed horizontal analysis of the chloroform-soluble components present. This information can aid in determining the degree of cure across the bondline, the migration of components, and the migration of plasticizers from the materials bonded together into the bondline. Acknowledgment
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The sectioning by microtomy of the bondline samples at the Harrington Arthritis Research Center in Phoenix, AZ, by Janson Emmanuel is gratefully acknowledged. Preparation of the bondline samples by the personnel of the Composites Materials & Process Department, Advanced Technology, Thiokol Corp., is also gratefully acknowledged.
Literature Cited
0.0%
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Figure 8. Comparison of the 96 sol.from mimtomy/NMR analysis with the 96 sol. from the standard 96 sol. analysis.
sample. These combined values are presented in Figure 7. The trends observed in both sets of values for the different bondline samples are very similar with the exception of the Epon 871 values. The relative amount of Epon 871 was greater for the samples analyzed by the standard % sol. method. This is probably due to the greater amount of prepreg resin from the Kevlar mat (which contains Epon 871 as a major ingredient) that can be obtained when the bondline samples are scraped as compared to microtomy slicing, which was terminated once the first Kevlar fibers were encountered. The % sol. values determined from the microtomy/NMR technique are compared to the % sol. values from the standard % sol. analysis in Figure 8. Again, the same trend among the samples is observed for each technique. The magnitude of the microtomy/NMR values are slightly larger.
Dartyge, J. M.; Ferrieres, C. Ultramicrotomy in the Transmission Electron Microscopy Study of Wear Transfer Films of CarbonFibre-Reinforced Polyimide Resin. Wear 1980,63,339-346. Hadad, D. K. Physical and Chemical Characterization of Epoxy Resins. In Epoxy Resins, Chemistryand Technology;May, C. A., Ed.; Dekker: New York, 1988. Jahn, H.; Guetzky, P. Chemical Analysis of Epoxides and Epoxy Resins. In Epoxy Resins, Chemistry and Technology;May, C. A., Ed.; Dekker: New York, 1988. Kruse, J. Electron Microscopical Determination of Rubber Blendability and the Visualization of Structure Domains. Kautsch. Gummi, Kunstst. 1976,29 (6), 321-328. Schaefer, H.; Hennemann, 0.D.; Rickel, J. Ultramicrotomy as a Preparation Method for Studies of Adhesive Joints. Sonderb. Prakt. Metattogr. 1986,17 (Metallographie), 369-376. Stangl, M.; Binder, K.; Tschamler, H. Thin Sections as an Aid~for the Evaluation of the Aging of Plastics. Chem. Kunstst.-AktueU 1976,30 (5), 217-220. Wolf, G. Preparation and Evaluation of Microtome Sections of Plastic Materials. Prakt. Metattogr.,Sonderb. 1978,9 (Metallogr. Keramogr.-Fortschr. Praeparationstech.), 188-196.
Received for review October 25,1990 Revised manuscript received March 19, 1991
